Electric drives generally have two components. On the one hand, the electric drive has an electric machine which converts mechanical energy into electrical energy or vice versa (electromechanical transformer). The electric machine may be operated both in generator mode as a generator and in motor mode as an (electric) motor. On the other hand, the electric drive has a power inverter or an inverter/rectifier circuit. The power inverter converts an electrical AC voltage into an electric DC voltage or vice versa (electric transformer). In most cases such an inverter has electric switches, often power semiconductors such as MOSFETs, and is actuated by a logic circuit. The power inverter and logic circuit are jointly also referred to as inverter.
For construction-related reasons, electric machines are usually connected via their housing to the engine block of the internal combustion engine in a conductive and permanent manner. The engine block represents the electric ground for the electric machine (engine ground). An excitation coil of the electric machine, which is usually incorporated into a vehicle electrical system, establishes a connection between the voltage-side and ground-side terminal of the electric machine. At the same time, an energy store, in particular a battery or a corresponding capacitor, is incorporated into a vehicle electrical system by its positive pole, and connected to a chassis of the motor vehicle as ground via its negative pole. Consumers in the vehicle electrical system are also grounded via the chassis (chassis ground).
The engine ground thus constitutes a relative ground potential for the electric drive. The chassis ground constitutes a reference ground potential for the vehicle electrical system. In order to relate the engine block and the chassis to the same ground potential, the engine block and chassis are electrically connected via a ground line or a ground strap. If an interruption occurs on this ground line or a break in this ground strap (hereinafter referred to as ground strap break), the negative reference point of the electric drive is undefined. The relative ground potential can therefore rise or drop in an uncontrolled manner.
A ground strap break may result in damage to the electric drive. An overvoltage may occur if a ground strap break occurs in a regenerative operation of the electric drive. A ground strap break during an operation of the electric drive as motor may lead to a critical voltage of the logic circuit, which could lead to a reset and loss of control over the electric drive. The logic circuit must have a voltage supply at all times. If the logic circuit loses its energy supply, a faulty or even destructive functionality of the logic circuit may be the result. In the special case of a ground strap break, the logic circuit is unable to achieve its functionality.
Therefore, it is desirable to provide a possibility for transferring an electric drive into a safe operating state in the event of a ground strap break.